The invention relates to gas diffusion electrodes used in catalyzed electrochemical processes. Fields of application are fuel cells of the proton-exchange membrane type and electrochemical reactors, e.g. or making chlorine/caustic soda. The invention also relates to a method of making such electrodes.
An electrode of the type used in proton-exchange membrane fuel cells has a diffusion zone and an active zone. In conventional manner, the diffusion zone can include a substrate such as a carbon or graphite cloth which provides the diffusion function for the purpose of distributing a gas it receives to the active zone, a current collector function, because of its ability to conduct electricity, and a structural function of imparting mechanical strength to the electrode. The active zone includes a catalyst such as platinum, which comes in contact with an electrolyte.
Such an electrode is described in document FR-2 744 840 A1. The diffusion zones is defined by a layer formed by a carbon cloth embedded in a hydrophobic material, poytetrafluoroethylene (PTFE) filled with carbon black, The active zone is formed by grains of optionally activated carbon, supporting particles of platinum in contact with a solid electrolyte, such as xe2x80x9cNafionxe2x80x9d from DuPont de Nemours.
Known electrodes of that type suffer from limitation phenomena. The activated carbon grains are tortuous to such an extent as to make it difficult for the fuel gas (H2) to gain access to the catalyst particles. The same limitation exists when extracting protons (H+) for migration through the electrolyte. Furthermore, the structure of the active zone which is constituted by accumulated carbon grains having a diameter lying in the range a few hundreds of nm to a few xcexcm, gives rise to a damaging ohmic drop.
In addition, the method of manufacturing those known electrodes includes heat treatment to sinter the PTFE, which treatment is performed after the active zone has been formed, and which can therefore spoil the catalyst.
Other documents relate to gas diffusion electrodes suitable for use in electrochemical processes that require the catalyst (Pt) to be fixed directly on activated carbon fibers.
This applies to the document by T. Yamamoto et al. xe2x80x9cElectrochemical reduction of CO2 in microporesxe2x80x9d, Studies in Surface Science and Catalysis, Vol. 114, 1998, XP-002119197. The diffusion zone of the electrode is formed by a mixture of acetylene black and of PTFE, while the active zone is formed of a mixture of activated carbon fibers carrying the metal catalyst, acetylene black, and PTFE. The result is shaped and subjected to heat treatment.
Reference can also be made to the document by J. H. Liew et al., xe2x80x9cCathode performance of AFC/acetylene black hybrid electrodes for phosphoric acid fuel cellxe2x80x9d, TANSO 1992 [No. 155], pp. 407-410, XP-002119196. A mixture of carbon black on which platinum has been fixed, of short fibers of activated carbon on which platinum has been fixed, and of PTFE is shaped and subjected to heat treatment so as to sinter the PTFE. In that case also, heat treatment is applied after the catalyst has been fixed. The presence of short fibers of activated carbon improves the properties of the electrode in terms of current density, with an optimum being obtained when the proportion by weight of activated carbon fibers carrying platinum is 20% and that of acetylene black carrying platinum is 80%.
A simplification of the above process for making an electrode for the same application is proposed in the document JP-64/014873 A which proposes using only short fibers (5 mm to 6 mm long) of activated carbon provided with catalyst. It is also proposed to do without the hydrophobic material (PTFE), assuming that the phosphoric acid wets the micropores of the fibers without closing off the macropores of the fiber substrate. However, that cannot always be transposed to other gas diffusion electrode applications using fibers and in which the presence of hydrophobic material remains necessary, at least in the diffusion zone.
An object of the invention is to propose gas diffusion electrodes enabling the operation and the efficiency of electrochemical devices in which they are used to be improved.
Another object of the invention is to propose a method of manufacturing gas diffusion electrodes which do not spoil the catalyst and which, in addition, make it possible to use the catalyst optimally.
According to the invention, in a gas diffusion electrode comprising a porous substrate of activated carbon fibers on which particles of a catalyst for an electrochemical reaction are fixed directly, the substrate is a cohesive fabric that is at least partially coated in a hydrophobic material.
The term xe2x80x9cactivated carbon fibersxe2x80x9d is used herein to mean continuous or discontinuous filaments of activated carbon, and preferably continuous filaments so as to favor electronic conduction.
The term xe2x80x9ccohesive fabric of activated carbon fibersxe2x80x9d is used herein to mean a fabric that inherently possesses mechanical properties that are sufficient not only to enable it to be handled during the process of manufacturing the electrode, but also to confer on it the mechanical strength required for its intended use.
The activated carbon fiber fabric is preferably a woven cloth. Other cohesive fabrics could be used, such as sheets of unidirectional fibers, possibly superposed in different directions, and bonded together, e.g. by needling. For convenience, in the description below, the term xe2x80x9cclothxe2x80x9d is used to designate a cohesive fabric regardless of whether or not it is woven.
The invention is remarkable in that the activated carbon fiber cloth performs simultaneously the functions of diffusing the gas admitted through the cloth, of conducting electrons, of providing a structural element that gives mechanical strength to the electrode, and of supporting the catalyst.
Advantageously, the cloth offers an array of microchannels to the admitted gas and leading to the particles of catalyst, thereby enabling the gas to react with each active portion supported by the cloth.
Since the activated carbon fiber cloth is organized in the form of an array of small diameter filaments, the tortuous nature and the ohmic drop are both smaller than is the case with the carbon grains used in the abovementioned prior art.
In addition, the activated carbon fiber cloth which is preferably mace of cellulose precursor fibers and more particularly of rayon precursor fibers, advantageously presents porosity characterized by pores of very small mean size, typically lying in the range 0.3 nm to 10 nm, thereby enabling maximum dispersion of the catalyst particles and enabling them to have a size that is optimal for the purpose of best operation of the electrode.
According to a feature of the activated carbon fiber cloth, it has a first face that is coated in a hydrophobic material such as PTFE, and a second face to which the catalyst particles are fixed.
In another aspect, the invention provides a method of making a gas diffusion electrode comprising the steps of supplying a substrate of activated carbon fibers, putting the substrate in contact with a precursor for an electrochemical reaction catalyst, and treating the precursor to obtain particles of catalyst fixed on the fibers of the substrate, the method being characterized in that an activated carbon fiber substrate is used in the form of a cohesive fabric having a first face and a second face, a portion of the fabric adjacent to its first face is coated in a hydrophobic material, and the catalyst is deposited on the fibers of the remaining portion of the fabric adjacent to its second face.
Advantageously, a controlled oxidation treatment of the activated carbon fiber cloth is performed prior to depositing the catalyst, so as to increase the concentration of functional groups constituting the surface chemistry of the activated carbon cloth.
The catalyst can be deposited by cation exchange or by liquid impregnation using a precursor salt for the catalyst. Such methods of depositing catalyst on activated carbon fibers are described in patent application WO 99/26721 A in the name of the Applicant. Depositing catalyst on activated carbon fibers by cationic exchange in order to obtain substrates having high level catalytic and electrocatalytic properties has also been envisaged in the document by P. P. Andonoglou et al., xe2x80x9cPreparation and electrocatalytic activity of rhodium modified pitch-based carbon fiber electrodesxe2x80x9d, Electrochimica Acta 44 (1998), pp. 1455-1465.
Advantageously, the precursor is a salt of the catalyst, and the treatment of the precursor comprises a step of reducing the salt by means of a gas admitted through the first face of the cloth.
All of the active catalyst obtained by such treatment is therefore necessarily accessible subsequently to a gas when the electrode is being used, thereby offering a maximum potential use of the catalyst. The electrode obtained in this way thus provides an array of microcharnels enabling the gases to reach each active portion supported by the activated cloth.
The activated carbon fiber cloth can be obtained in various ways.
A first possibility consists in using a carbon precursor fiber cloth, in particular a cellulose cloth, and more particularly a rayon cloth, and in implementing a stage of heat treatment for carbonizing the carbon precursor. The resulting carbon fiber cloth is activated by heat treatment under an oxidizing atmosphere.
A second possibility consists in starting with a carbon precursor fiber cloth and impregnating it with a composition suitable, after carbonization, for obtaining an activated carbon fiber cloth directly. The carbon precursor is preferably a cellulose, and more particularly rayon. Impregnation is performed using a composition containing an inorganic ingredient with the function of promoting dehydration of the cellulose, e.g. phosphoric acid.
The activated carbon fiber cloth can be coated in part in a hydrophobic material in various ways: the first face of the cloth can be coated, the cloth can be rolled together with a sheet of hydrophobic material placed on the first face of the cloth, a sheet of hydrophobic material can be stuck onto the first face of the cloth, or a composition containing the hydrophobic material can be sprayed onto the first face of the cloth, while the second face is advantageously heated, e.g. to a temperature in the range 120xc2x0 C. to 160xc2x0 C., so as to prevent the composition from fixing on that face.
The hydrophobic material is sintered by heat treatment performed prior to depositing the catalyst, and thus incapable of affecting the catalyst.
Also advantageously, the method is implemented on a traveling strip of activated carbon fiber cloth from which electrodes are subsequently cut out.
In the detailed description given below by way of non-limiting indication, reference is made to the accompanying drawings, in which:
FIG. 1 is a highly diagrammatic view of a first embodiment of a gas diffusion electrode of the invention;
FIG. 2 shows the successive steps in a method of making the electrode of FIG. 1;
FIG. 3 is a highly diagrammatic view of a second embodiment of a gas diffusion electrode of the invention;
FIGS. 4A to 4C show an implementation of the method of the Invention on a traveling strip of activated carbon fiber cloth;
FIG. 5 is a transmission electronic microscope picture showing particles of catalyst on the surface of an activated carbon fiber in an electrode made in accordance with the invention; and
FIG. 6 is a histogram showing the size distribution of particles of catalyst fixed on the activated carbon fibers of an electrode made in accordance with the invention.